1. Field of the Invention
The present invention relates to a balanced high-frequency filter, an antenna duplexer, a balanced high-frequency circuit and a communication apparatus.
2. Related Art of the Invention
In recent years, with the development of mobile communications, there have been expectations for improvements in performance and reductions in size of devices to be used for mobile communication. As a filter for use in a radio frequency (RF) stage, surface acoustic wave filters have been widely used. Also, in recent years, there have been expectations for filters using film bulk acoustic resonators (FBAR). Balancing of such filters and semiconductor elements used in RF stages has been pursued for the purpose of improving noise characteristics, for example, in terms of crosstalk between devices and there is a demand for improved balance characteristics.
A conventional balanced high-frequency device is described below. FIG. 28 shows a configuration of a conventional balanced high-frequency device 2801. The balanced high-frequency device 2801 is constituted by an input terminal IN serving as a unbalanced input/output terminal and output terminals OUT1 and OUT2 serving as balanced input/output terminals.
Moreover, in the case of a balanced high-frequency device, impedance matching is necessary. FIGS. 29(a) and 29(b) shows configurations of conventional balanced high-frequency devices respectively having a matching circuit. In FIG. 29(a), a balanced high-frequency device 2901 is constituted by an input terminal IN serving as an unbalanced input/output terminal and output terminals OUT1 and OUT2 serving as balanced input/output terminals. Moreover, a matching circuit 2902 is connected between the output terminals OUT1 and OUT2. Moreover, in FIG. 29(b), a balanced high-frequency device 2903 is constituted by an input terminal IN serving as an unbalanced input/output terminal and output terminals OUT1 and OUT2 serving as balanced input/output terminals. Furthermore, matching circuits 2904 and 2905 are connected between the output terminals OUT1 and OUT2 and ground planes respectively. This type of the matching circuit is used to match a balanced high-frequency device with the characteristic impedance of a balanced input/output terminal.
As an example of the above balanced high-frequency device, a conventional surface acoustic wave filter is described below. FIG. 30 shows a block diagram of an surface acoustic wave filter 3001 having a balanced input/output terminal. In FIG. 30, the surface acoustic wave filter 3001 is constituted on a piezoelectric substrate 3002 by first, second, and third inter-digital transducer electrodes (hereafter respectively referred to as IDT electrode) 3003, 3004, and 3005 and first and second reflector electrodes 3006 and 3007. One-hand electrode finger of the first IDT electrode 3003 is connected to an output terminal OUT1 and the other-hand electrode finger of the first IDT electrode 3003 is connected to an output terminal OUT2. Moreover, one-hand electrode fingers of the second and third IDT electrodes 3004 and 3005 are connected to an input terminal IN and the other-hand electrode fingers of the electrodes 3004 and 3005 are grounded. By using the above configuration, it is possible to realize an surface acoustic wave filter having an unbalanced-balanced input/output terminal. Moreover, in the case of the surface acoustic wave filter in FIG. 30, impedances of the input and output terminals are respectively designed as 50 Ω.
Moreover, a conventional surface acoustic wave filter is described below as an example of a balanced high-frequency device having a matching circuit. FIG. 31 shows a block diagram of surface acoustic wave filter 3101 having a matching circuit. In FIG. 31, the surface acoustic wave filter 3101 is constituted on a piezoelectric substrate 3102 by first, second, and third inter-digital transducer electrodes (hereafter respectively referred to as IDT electrode) 3103, 3104, and 3105 and first and second reflector electrodes 3106 and 3107. The first IDT electrode 3103 is divided into two divided IDT electrodes. One electrode finger of a first divided IDT electrode 3108 is connected to an output terminal OUT1, one electrode finger of a second divided IDT electrode 3109 is connected to an output terminal OUT2, and the other-hand electrode fingers of the first and second divided IDT electrodes are electrically connected. Moreover, one-hand electrode fingers of the second and third IDT electrodes 3104 and 3105 are connected to an input terminal IN and the other-hand electrode fingers of the electrodes 3104 and 3105 are grounded. Furthermore, an inductor 3110 is connected between output terminals as a matching circuit. By using the above configuration, it is possible to realize surface acoustic wave filter having an unbalanced-balanced input/output terminal. Furthermore, in the case of the surface acoustic wave filter in FIG. 31, impedances of input and output terminals are designed as 50 Ω for the input side and as 150 Ω for the output side. Therefore, the filter has an impedance conversion function.
FIGS. 32(a) to 32(c) show characteristic diagrams of a conventional surface acoustic wave filter of a 900-MHz band shown in FIG. 30. In FIGS. 32(a) to 32(c), FIG. 32(a) shows a passing characteristic, FIG. 32(b) shows an amplitude balance-characteristic in a pass band (from 925 up to 960 MHz), and FIG. 32(c) shows a phase balance-characteristic in a pass band. From FIG. 32, it is found that the amplitude balance-characteristic greatly deteriorates from −0.67 dB to +0.77 dB and the phase balance-characteristic greatly deteriorates from −6.3° to +9.4° in each pass band.
In this case, the amplitude balance-characteristic denotes the difference between the signal amplitude of the input terminal IN and output terminal OUT1 and the signal amplitude of the input terminal IN and output terminal OUT2. When the difference becomes zero, the balance-characteristic does not deteriorate. Moreover, the phase balance-characteristic denotes a shift of the difference between the signal phase of the input terminal IN and output terminal OUT1 and the signal phase of the input terminal IN and output terminal OUT2 from 180°. When the difference becomes zero, the balance-characteristic does not deteriorate.
With the above-described balanced high-frequency device and the surface acoustic wave filter described as an example of the balanced high-frequency device, however, there has been a problem that a deterioration in balance characteristics considered one of important electrical characteristics of the device is large.
Also, the balanced high-frequency device in the conventional art is a phase-shifting circuit used to improve the balance characteristics by considering the characteristics in the pass band, and the characteristics outside the pass band have not been taken into consideration. In a case where a balanced high-frequency element provided as the balanced high-frequency device is connected to the input side of a semiconductor device, not only the characteristics in the pass band but also the characteristics outside the pass band are important. In a case where a balanced high-frequency element is used for a receiving filter in particular, characteristics in a transmission frequency band are important as well as those in a reception frequency band. With the conventional balanced high-frequency device, however, there is a problem that the amount of leakage of common-mode signal components to the balanced output terminals is large.